An acceleration sensor which is one of MEMS (micro electro mechanical systems) sensors is made up of a weight (movable part) and a support beam (elastic deformation part), and converts acceleration exerted on the weight into an electric signal. The MEMS sensors of this type include a capacitance sensor that detects the acceleration as a capacitance change between electrodes configuring a capacitor. The capacitance sensor captures the acceleration exerted on the weight as a change in the capacity or the like, and converts the acceleration into an electric signal through an LSI circuit as an output of the MEMS sensor.
For example, in an attitude control system that suppresses the sideslip and spin of a vehicle to assist safe travel, the MEMS sensors such as an angular velocity sensor for detecting an angular velocity, and an acceleration sensor for detecting accelerations in a longitudinal direction and a lateral direction are used as key devices. In the attitude control system, an output of an engine and a braking force of a brake are controlled on the basis of the outputs from the angular velocity sensor and the acceleration sensor to suppress the sideslip and spin of the vehicle. From this viewpoint, there is a high possibility that a failure of the angular velocity sensor or the acceleration sensor leads to an accident, and when the angular velocity sensor or the acceleration sensor fails, this fact needs to be immediately announced.
JP-A-2007-248328 discloses an acceleration sensor including: a detection element having two movable electrodes that are movable due to acceleration and two detection electrodes which are disposed to face those respective movable electrodes; a drive circuit that inputs an AC bias voltage signal to the movable electrodes; and a detection circuit that outputs an acceleration signal on the basis of two detection signals output from the two detection electrodes. In the detection circuit of this acceleration sensor, there is provided failure detecting means that, apart from a reference voltage signal that is inputted to two amplifiers, a detection voltage signal is inputted to those two amplifiers from a detection voltage supply part to detect a failure on the basis of a detection differential signal output from a differential detector.
JP-A-Hei05 (1993)-281256 discloses that a diagnosis signal having a low frequency is superimposed on an AC signal supplied for the purpose of detecting a capacitance of the detection electrodes to change the capacitances between the movable electrodes and the fixed electrodes. JP-A-Hei05 (1993)-281256 also discloses that whether the output resultantly generated from the acceleration sensor oscillates, or not, can be observed to determine whether there is a failure in the acceleration sensor, or not, while detecting the acceleration which is a signal.
JP-A-Hei05 (1993)-281256 further discloses that because the diagnosis signal having the lower frequency allows the output to oscillate with the amplitude of the degree that does not affect the measurement of acceleration, constant diagnosis for diagnosing the presence/absence of the failure while detecting the acceleration is enabled. JP-A-Hei05(1993)-28125 further discloses when the diagnosis signal (changeover signal of the diagnosis types) is a low level, high acceleration diagnosis that allows the movable electrodes to largely forcedly oscillate is also enabled.